Bubble-Sweep down Study and Mitigation for Improved ADCP

Data Quality

Credits go to:• Mr. Bob Fratantonio - Department of Ocean Engineering

University of Rhode Island

• Dr. Thomas Rossby– Graduate School of Oceanography

University of Rhode Island

• Dr. Charles Flagg– School of Marine and Atmospheric Sciences

Stony Brook University

• Dr. Stephan Grilli– Department of Ocean Engineering

University of Rhode Island

• National Science Foundation (NSF)• Smyril Line

The M/F Norröna• Build year/Shipyeard: 2003 / Flendern Werft AG, Lübeck • Ship contract price: € 93,4 mill.• Length over all: 165,74m• Breath: 30,00m• Draft: 6,30 m• Dwt: 6.350• GT: 35.966• NT: 15.922• Cabins: 318 (1012 beds)• Passenger capacity: 1482• Crew: 118• Cars: 800 or Trailers: 130• Lane m.: 1830• Cargo capacity: 3.250 tonnes• Service speed: 21 knots• Main engines: 30.000 BHP• Bow Thrusters: 4.755 BHP• Helicopter pad: On top deck at the ferries stern• Stabilizers: 1 pair of stabilizers

ADCP• 75 kHz RD Instruments Ocean Surveyor• Installed in a 1-week dry dock period in January

2006 in Hamburg, Germany• Cable runs 8 decks to the DAQ system• ADCP is mounted 60 m from bow

Dry Dock

The Bubble Fairing

Previous Results using the Bubble Fairing

The Problem

• An ADCP system was installed on the M/F Norröna in January 2006 in Hamburg, Germany

• Instrument was functioning properly, but the data was spotty and poor

• Data improved as M/F Norröna passed through fjords towards Bergen, Norway

• As the ferry entered open seas, the acoustic backscatter amplitude became erratic and of poor quality

Candidates for Source of Problem

• Internal machinery-generated vibration

• Propeller noise• Electronic interference due

to the long length of cable that necessarily ran along-side some of the ship's power cables

• Bubble Sweepdown– Breaching of the bow-thruster

openings?

CritterCam• Greg Marshall at the

National Geographic Society loaned us the CritterCam

• Features– Autonomous– Records Internally– Diver Deployable

• Records 1 minute of video every 4 hours

• Permanent magnets attach camera to the hull

CritterCam Results• Best results come from

videos taken during daylight hours

• Bubble clouds are produced in the turbulent bow wave as the ferry pitches up and down– Clouds approach lens at fairly

regular intervals• Using the height of the fairing

(21 cm) as reference, one can estimate the thickness of the clouds seen in the video as roughly 30 cm thick

CritterCam ResultsIf the video clip does not play automatically, it can be accessed by

clicking the following link: http://www.unols.org/meetings/2009/200903fic/bubblesweep.AVI

Windows users may need to download the free divx codec to view the video clip. The download is available at: http://

www.divx.com/en/products/software/windows/divx

Cosmos Floworks• Computational Fluid Dynamics were performed to

address the following questions…– Can the shape of the fairing be improved to reduce the

stagnation pressure at the leading edge of the fairing?– Can the addition of rails placed ahead of the fairing produce

significant upwelling to bring bubble-free waters from depth up to the face of the transducer?

• Used Cosmos Floworks CFD package– Fully embedded in Solidworks– Easy to use

• Computations were performed on a Dell Optiplex 755 running Windows XP Professional– 8 GB of RAM– Intel® Core™ 2 Duo CPU E6850 @ 3.00 GHz

Rails

• The next step was to investigate the influence of rails upstream of the fairing

• Rails were modeled after a hyperbolic tangent functiony = A * tanh(x) + b

• A systematic approach was taken to optimize the parameters of the rails

• Once the rails were optimized, the rail-fairing interaction could be simulated and studied

Varying Opening Width

• The first parameter to change was the opening between the two rails.

• The slope of the rails remained constant and only the opening was changed

•Equation

y= A*tanh(x)+b

Varying b changes the width of the opening between rails

Some Results

Vortices Generated by Rails

Very encouraging!

The rails do appear to generate upwelling

* Note this figure is upside down

Varying Slope• The next parameter to change was the slope

• The opening between rails remained constant and only the slope was changed

•Equation

y= A*tanh(x)+b

Varying A and offsetting b the same amount changes the width of the opening between rails

Final Rail Profile• The rails were shortened from their original 4 meters of length (in the x-dir) to 2 meters

• The opening was optimized as the same width as the fairing, ~0.5 meters

• The height of the rails matched that of the fairing, ~20cm

Rail – Fairing InteractionPlanview of Z-Velocity

Rails are set 10 meters upstream of the fairing

Chines• Can we simplify the rails even more?

• Straight rails (chines) were of interest due to their simplicity

• Easy and less expensive to manufacture and install

• But do they perform as well as the rails?

• Use approximately same slope as the hyperbolic tangent rails

Chines vs. Rails

Rails Chines

Particle Trajectories - Chines

Water particles released downstream 0.5 meters below the hull starting from the centerline and spanning 1 meter starboard

Particle Trajectories - Rails

Particle Displacement Profile (Y-Z)

The rails and chines create a similar swath

Sketch of New Fairing/Rails Position

• The fairing was moved closer to the centerline of the ship with the hyperbolic tangent rails ~10 meters upstream

The Rails

Photos of the rails just before the ship was refloated, courtesy of Eike Bayer, the Blohm

and Voss project director.

Plans for the Future

• Still having difficulty collecting good ADCP data– Not entirely sure why– Lack of Zooplankton for acoustic backscatter?

• Would like to use the camera to get visual evidence of whether the rails are successfully creating local upwelling